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AR# 36260

Timing - Multi-Cycle constraint not overriding PERIOD constraint

Description

An issue was seen with a design containing with a FROM-TO that does not override a PERIOD constraint. The problem relates to asynchronous DSP blocks only, so will not be a happen frequently.

The problem was due to the FROM timegroup containing a number of DSP elements that are intermediate points, thus acting as asynchronous startpoints.

In other words, if you have the following: if src_FF->DSP1->DSP2->DSPn->dest_FF, and if DSPn is contained in the FROM timegroup of the MAXDELAY, then the path from DSPn->dest_FF will be found first and returned, with no further expansion backward beyond DSPn.

Solution

This is expected behavior, as when a group of sources are specified then when one is found, timingdoes not continue analysis. For example,if Aand B are sources and a path is found to B, even if the path continues to A, it is not analyzed.